Variable-effect lighting systems are commonly used for advertising, decoration, and ornamental or festive displays. Such lighting systems frequently include a set of coloured lamps packaged in a common fixture, and a control system which controls the output intensity of each lamp in order to control the colour of light emanating from the fixture.
For instance, Kazar (U.S. Pat. No. 5,008,595) teaches a light display comprising strings of bicoloured LED packages connected in parallel across a common DC voltage source. Each bicoloured LED package comprises a pair of red and green LEDs, connected back-to-back, with the bicoloured LED packages in each string being connected in parallel to the voltage source through an H-bridge circuit. A control circuit, connected to the H-bridge circuits, allows the red and green LEDS to conduct each alternate half cycle, with the conduction angle each half cycle being determined according to a modulating input source coupled to the control circuit. However, the rate of change of coloured light produced is restricted by the modulating input source. Therefore, the range of colour displays which can be produced by the light display is limited.
Phares (U.S. Pat. No. 5,420,482) teaches a controlled lighting system which allows a greater range of colour displays to be realized. The lighting system comprises a control system which transmits illumination data to a number of lighting modules. Each lighting module includes at least two lamps and a control unit connected to the lamps and responsive to the illumination data to individually vary the amount of light emitted from each lamp. However, the illumination data only controls the brightness of each lamp at any given instant. Therefore, the lighting system is not particularly well suited to easily producing intricate colour displays.
Murad (U.S. Pat. No. 4,317,071) teaches a computerized illumination system for producing a continuous variation in output colour. The illumination system comprises a number of different coloured lamps, a low frequency clock, and a control circuit connected to the low frequency clock and to each coloured lamp for varying the intensity of light produced by each lamp. However, the rate of change of lamp intensity is dictated by the frequency of the low frequency clock, and the range of colour displays is limited.
Gomoluch (GB 2,244,358) discloses a lighting control system which includes a lighting control unit, and a string of light units connected to the lighting control unit. The lighting control unit includes a DC power supply unit, a microprocessor, a read-only memory containing display bit sequences, and switches for allowing users to select a display bit sequence. Each light unit includes a bi-coloured LED, and data storage elements each connected in parallel to the DC power output of the lighting control unit and in series with data and clock outputs of the microprocessor. The microprocessor clocks the selected bit patterns in serial fashion to the storage elements. The data storage elements received each data bit, and illuminate or extinguish the associated LED.
However, Gomoluch requires that complex light units be used. Therefore, there remains a need for a relatively simple variable-effect lighting system which allows for greater variation in the range of colour displays which can be realized.